The high efficiency of switching power converters such as flyback converters has led to their virtual universal adaption as the battery charger for mobile devices. In a flyback converter, a controller controls the cycling of a power switch transistor that connects between the transformer's primary winding and ground. A rectified AC mains voltage drives the primary winding current when the power switch is cycled on. The rectified AC mains voltage can be several hundred volts such that it can stress the power switch transistor. To minimize the switching stress for the power switch transistor, both quasi-resonant (valley-mode switching) and zero-voltage switching techniques are known. For example, it is known to employ valley switching techniques with regard to the resonant oscillation of the drain voltage for the power switch transistor when it is cycled off. The peak voltages for the resonant oscillation can be relatively robust (as much as 200 V or higher) whereas the minimum voltages (the valleys in the resonant oscillations) are much lower. Valley-mode switching thus involves the detection or prediction of a valley in the resonant oscillations so that the power switch transistor may be switched on at the valley time.
Although valley-mode switching thus lowers the voltage stress on the power switch transistor, note that the valley voltages are not zero but may range from 20 V or even higher such as up to 250 V. This relatively high drain voltage is then discharged to ground when the power switch transistor is cycled on, which lowers efficiency. A more power-efficient alternative to valley-mode switching is zero-voltage-switching (ZVS), which may also be denoted as active clamp operation. In active clamp operation, the leakage energy in the transformer is stored and reclaimed in an active clamp capacitor that is coupled to a terminal for the power switch transistor through an active clamp switch transistor. The active clamp switch transistor is cycled on at the peak of the resonant oscillations, whereupon the drain voltage for the power switch transistor is discharged to ground as the leakage energy is reclaimed. An active clamp architecture thus has no stressing switches at the on-time of the power switch transistor because the on-time is timed to occur when the drain voltage is discharged.
Although active clamp operation is thus advantageous, switching of the active clamp switch requires an appropriate driver. It is conventional to power the driver of the active clamp switch transistor using charge from a boot strap capacitor that in turn is charged by a power supply voltage VCC from a power supply capacitor. An example flyback converter 100 is shown in FIG. 1. A controller U1 controls the switching of a power switch transistor S1 to regulate an output voltage stored on an output capacitor Cout. Power switch transistor S1 has a drain terminal connected to a primary winding of a transformer T so that an input voltage Vin forces a magnetizing current to flow in the primary winding. During this on-time of power switch transistor S1, an output diode D3 prevents a current from flowing in a secondary winding of the transformer. This rectification may also be performed by a synchronous rectifier switch transistor in alternative implementations. When controller U1 cycles off power switch transistor S1, the output diode D3 becomes forward biased so that the secondary current flows to charge the output capacitor Cout with the output voltage. The drain of power switch transistor S1 is charged high while power switch transistor S1 is off. Similarly, an auxiliary winding (not illustrated) is also charged high when power switch transistor S1 is off. To harvest this energy to support the power supply voltage VCC, the auxiliary winding (Aux) couples through a current-limiting resistor R1 and a power supply diode D2 to charge a power supply capacitor VCC with the power supply voltage VCC.
The power supply voltage VCC also powers a driver Dr for an active clamp switch transistor S2 that couples between the drain of power switch transistor S1 and an active clamp capacitor Ca that in turn couples to the input power rail supplying the input voltage Vin. In particular, a diode D1 couples the power supply voltage VCC to a bootstrap capacitor CB. The resulting voltage from the bootstrap capacitor CB powers driver Dr so that active clamp operation may be achieved. But as the switching frequency for power switch transistor S1 reduces during low load operation, the charging of power supply capacitor VCC is reduced so that the power supply voltage drops accordingly. The voltage across the bootstrap capacitor for powering driver Dr may then be too low such that active clamp operation is lost, causing additional switching loss, voltage spike, and electromagnetic interference (EMI) issues. In particular, the leakage inductance energy that would ordinarily be released may accumulate on active clamp capacitor Ca. The resulting voltage rise across active clamp capacitor Ca may damage it and cause safety issues. Additional circuitry is thus needed to avoid breakdown of active clamp capacitor Ca during low-frequency operation, which adds to component count and increases cost.
Accordingly, there is a need in the art for improved active clamp operation for flyback converters.